#3. Box Jellyfish
You will find it in the Indo-Pacific waters north of Australia. These transparent, nearly invisible invertebrates are considered by the National Oceanic and Atmospheric Administration as the most venomous marine animal in the world. Their namesake cubic frames contain up to 15 tentacles at the corners, with each growing as much as 10 feet long, all lined with thousands of stinging cells. It contain toxins that simultaneously attack the heart, nervous system, and skin cells. It is so venomous it will kill you within two minutes if you receive two metres or more tentacle contact. If a tentacle touches the skin it results in a very painful sting.